Semiconductor devices are fabricated through various processes including dry etching and deposition processes, many of which resort to plasma techniques. In the plasma processes, highly reactive, corrosive halogen-based gases such as fluorine or chlorine-based gases are often used for the purposes of etching, deposition and cleaning.
For more efficient semiconductor fabrication, it is desired to increase the operation efficiency of these plasma systems. The operation efficiency of plasma systems can be increased by reducing the downtime of systems, which is achieved, for example, by reducing the frequency of wet cleaning of chamber components.
One means for reducing the frequency of wet cleaning is plasma cleaning. That is, any foreign deposits on the reaction chamber resulting from a dry etching or deposition process are removed by applying a suitable gas plasma to reaction products for decomposition or sublimation thereof, and exhausting the decomposed or sublimated products. The plasma cleaning is effective in reducing the frequency of wet cleaning to some extent. When such plasma cleaning is performed, it is essential to place a dummy wafer on a wafer chuck including the lower electrode within the chamber so as to prevent the lower electrode from direct exposure to the plasma. Even after the plasma cleaning, it is also necessary to hold the dummy wafer within the chamber for the purpose of positively expelling the particles remaining on the inner wall and other members of the chamber and the cleaning gas.
For consistent fabrication of semiconductor devices, the processing system is desired to keep a stable plasma state. In the plasma system, however, the temperature is unstable because plasma treatment entails heat accumulation so that the system interior undergoes a temperature rise or variation at the initial stage of operation. Thus, at the initial stage of operation, a plasma treatment equivalent to the actual process, which is known as dummy treatment, is carried out on a plurality of dummy wafers for the purpose of minimizing a temperature change and keeping the system temperature stable. The dummy treatment is implemented not only for the purpose of stabilizing the system temperature, but also for the purposes of stabilizing the processing atmosphere and pressure prior to execution of etching treatment on substrates, testing system operation, and cleaning and seasoning (or aging) after cleaning. The dummy treatment is also implemented for determining the process conditions for a lot of substrates.
Since process parameters of the system, especially the etching rate of dry etching process, remain unstable immediately after power-on, dummy treatment is also carried out for the purpose of stabilizing the system performance. Also in this case, it is essential to place a dummy wafer within the chamber so as to prevent the lower electrode from being damaged by the plasma treatment.
Such dummy wafers are required to have high corrosion resistance and strength because they are brought in contact with corrosive gases and plasmas. Dummy wafers are generally made of silicon, quartz or the like.
More rigorous conditions are now employed for achieving the goal of improved productivity. For example, cleaning gases of higher corrosive nature are used to further reduce the cleaning time, rapid heating is used to reduce the heating time, and so on.
While silicon wafers or dummy wafers in the form of silicon wafers having a silicon oxide coating formed thereon and quartz wafers are used in the prior art, they have insufficient resistance to highly corrosive cleaning gases and etching gases and fail to inhibit dusting or contamination. These wafers are susceptible to thickness reduction by the cleaning gases and etching gases.
To solve the above and other problems, dummy wafers of alumina ceramics and dummy wafers of yttria-alumina compound-based ceramics (JP-A 2003-86475) were proposed. Undesirably alumina forms aluminum fluoride particles when contacted with fluorine-based gases such as SF6, CF4, CHF3, ClF3, HF, and C2F8. Additionally, ceramic wafers of alumina or yttria-alumina compounds are expensive because of many problems including a very long time of sintering, a long time of heating and cooling, low yields, and difficulty of productivity improvement. JP-A 9-45751 also discloses a dummy wafer.